You can grant or deny SMTP virtual server access to specific
users or groups, which you specify by IP address or domain name. By
default, the SMTP virtual server is accessible to all IP addresses.
You can set restrictions by specifying a single IP address, a group
of addresses using a subnet mask, or a Windows 2000 domain
name.

Tip If you choose to grant or
deny access based on domain name, you will get a warning window
informing you that a Domain Name System (DNS) reverse lookup on
each connection will be necessary. Reverse DNS lookups are
expensive and could tax your system, so be sure your system can
handle the additional processing if you select this option.